bass is a table-driven, patching cross assembler.
Unlike a traditional assembler, there is no linker stage: assembly code is patched directly onto existing images. It is primarily intended for modifying existing binaries; although it is fast enough to build complete programs from scratch as well.
Currently, it supports the WDC 65816 and Sony SPC700 CPUs.
bass v15 (source code)
bass is licensed under the GNU GPLv3 license.